In a motor having a brush, electricity is supplied to the motor when the brush contacts with a commutator. A coil wound around a core of a rotor is connected to the commutator. When electricity is supplied to the coil, the rotor is rotated by attracting/repulsive force from a permanent magnet provided in a housing and facing the rotor.
In the case of motors having structure described above, the brush and the commutator are slidably contacted together when the motor is under operation. Accordingly, the contact surfaces tend to wear. In order to prevent wear generations of brushes for motors, various considerations for brush materials and brush hardness were made.
In particular, for a brush motor material utilized in a brush motor for vehicles, a conventional metal-graphite brush material is described in JP2001-298913A. According to the document, longevity of brushes is considered. The metal-graphite brush material is manufactured as follows. First, carbon particles, copper particles, and a binding agent are mixed. Next, the mixture is sintered.
One example of a method for manufacturing a metal-graphite brush material will be explained precisely as follows. First, natural carbon particles are mixed with phenol resin solution as a binder and kneaded. Then, the mixture is formed into particles of predetermined shape. The obtained carbon particles are mixed with copper powder in the amount corresponding to current density to be applied to the brush and solid lubricant of required amount. Then, the mixture is formed into a predetermined shape. Then, the formed mixture is sintered in non-oxidative atmosphere in which oxygen is excluded. In this case, phenol resin coatings formed on surfaces of carbon particles are carbonized into amorphous carbon by reduction sintering. The amorphous carbon plays a role for binding carbon particles. Then, during sintering, carbon dioxide and vapor are vaporized from organic compounds originating in the phenol resin. Accordingly, many pores are formed on the surface and inside the sintered body.
However, it is known that the conventional motor brush material tends to generate spark discharge.
For example, considering the case of a metal-graphite brush material, the brush slides along the commutator and contacts therewith. From the fact that the brush and the commutator have some degree of surface roughness, the brush and the commutator can be assumed to be in contact with each other through three spots in microscopic view, the spots changing with time according to the condition of sliding. Electric field is applied to the brush through the three spots. The electric field causes to separate π electrons from the carbon particles of metal-graphite brush material. As a result, charge is induced in the metal-graphite brush material. Then, the induced charge transfers toward material of higher electric conductivity, in the case of the metal-graphite brush material, copper powder. At this time, because the copper powder has small capacity for storing charge, charge having transferred toward copper powder discharges toward outside the copper powder, which induces spark discharge.
Then, the discharge phenomena in which charge is emitted raises temperature near cores of spark discharge abruptly. Therefore, volumes of copper powder and carbon particles are abruptly expanded. Thus, bound between the copper powder and the carbon particles are broken caused by difference between volume expansion rates of the copper powder and the carbon particles. Further, copper powder has a sublimation point lower than that of carbon. Therefore, copper powder sublimates before carbon sublimates. Thus, volume reduction of copper powder caused by sublimation breaks bonds between copper powder and carbon particles. Accordingly, copper powder can easily drop from carbon particles, which causes easiness of wear of the brush. Further, noise signals are generated corresponding to the amount of charge emitted during spark discharge.
A need thus exists for a motor brush material utilized for a motor not easily generating spark discharge which can cause wear of a brush, and a method for manufacturing the motor brush material.